This invention relates to electron beam lithography apparatus used for the manufacture of semiconductor integrated circuits.
Electron beam exposure tools have been used for lithography in semiconductor processing for more than two decades. The first e-beam exposure tools were based on the flying spot concept of a highly focused beam, raster scanned over the object plane. The electron beam is modulated as it scans so that the beam itself generates the lithographic pattern. These tools have been widely used for high precision tasks, such as lithographic mask making, but the raster scan mode is found to be too slow to enable the high throughput required in semiconductor wafer processing. The electron source in this equipment is similar to that used in electron microscopes, i.e. a high brightness source focused to a small spot beam.
More recently, a new electron beam exposure tool was developed based on the SCALPEL (SCattering with Angular Limitation Projection Electron-beam Lithography) technique. In this tool, a wide area electron beam is projected through a lithographic mask onto the object plane. Since relatively large areas of a semiconductor wafer (e.g. 1 mm2) can be exposed at a time, throughput is acceptable. The high resolution of this tool makes it attractive for ultra fine line lithography, i.e sub-micron.
The requirements for the electron beam source in SCALPEL exposure tools differ significantly from those of a conventional focused beam exposure tool, or a conventional TEM or SEM. While high resolution imaging is still a primary goal, this must be achieved at relatively high (10-100 xcexcA) gun currents in order to realize economic wafer throughput. The axial brightness required is relatively low, e.g. 102 to 104 Acmxe2x88x922srxe2x88x921, as compared with a value of 106 to 109 Acmxe2x88x922srxe2x88x921 for a typical focused beam source. However, the beam flux over the larger area must be highly uniform to obtain the required lithographic dose latitude and CD control.
A formidable hurdle in the development of SCALPEL tools was the development of an electron source that provides uniform electron flux over a relatively large area, has relatively low brightness, and has an electron emitter with a sufficient lifetime to avoid excessive downtime. Lanthanum hexaboride (LaB6) emitters in a modified Wehnelt electron gun arrangement were found to be promising for this application, and the first SCALPEL tools were built with these electron sources. Efforts to improve the uniformity of the electron emission profile over the surface of the LaB6 have continued, but with limited success. Replacement of the LaB6 emitter with a simple tantalum disk was found to improve the surface emission uniformity and stability. While SCALPEL systems are regarded as highly successful fine line lithographic exposure tools, efforts continue toward improving the efficiency and uniformity of the electron beam source.
We have developed a new electron beam source for SCALPEL systems which uses an electron beam shaping element to smooth the electron beam profile of the primary emitting surface over the wide emission area. The beam shaping element is a mesh grid that is installed over the opening in the Wehnelt gun. The mesh grid is an equipotential screen with each aperture in the screen acting as a separate Wehnelt emitter with its own funnel-shaped electrical field. The result is a highly uniform wide area electron beam that is ideally suited for SCALPEL tools.
The optical analog of the proposed gridded Wehnelt is a fly""s eye lens, or scatter plates or diffusers, which transform non-uniform light beams into uniform beams in optical illumination systems.
The invention will be described more specifically in the following detailed description which, taken with the drawing, will provide a greater understanding of the features that distinguish this invention from conventional electron beam sources.